1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and an electrophotographic apparatus, process cartridge and method using the photoreceptor.
2. Discussion of the Background
An electrophotographic method is typically one of an image forming method of charging a photoconductive photoreceptor in a dark place with, e.g., a corona discharge; irradiating the photoreceptor with imagewise light; forming an electrostatic latent image thereon by selectively scattering a charge on the irradiated part thereon; and developing the latent image with a toner including a colorant such as dyes and pigments and a binder such as polymers to form a visual toner image.
In such an electrophotographic method, the photoreceptor requires the following basic properties:
(1) the photoreceptor can be charged so as to have a proper potential in a dark place;
(2) the charge scarcely scatters in the dark place; and
(3) the charge can immediately scatter when the photoreceptor is irradiated.
Conventionally, as photoreceptors for use in the electrophotographic method, a photoreceptor including a photosensitive layer mainly formed from selenium or a selenium alloy on an electroconductive substrate; a photoreceptor including an inorganic photoconductive material such as zinc oxide and cadmium sulfide in the binder; a photoreceptor using an organic photoconductive material such as poly-N-vinylcarbazole and trinitrofluorenone, or azo pigments; and a photoreceptor using an amorphous silicone material are typically known. However, recently organic electrophotographic photoreceptors are widely used because of their low production costs, high flexibility of their designs and low pollution.
As the organic electrophotographic photoreceptors, a photoconductive resin type photoreceptor typified by polyvinylcarbazole (PVK); a charge transfer complex type photoreceptor typified by PVK-TNF (2,4,7-trinitrofluorenone); a dispersed pigment type photoreceptor typified by phthalocyanine-binder; and a functionally-separated photoreceptor using a combination of a charge generation material and a charge transport material are known. Particularly, the functionally-separated photoreceptor attracts attention.
The electrostatic latent image forming mechanism in the functionally-separated photoreceptors is as follows: when light irradiates a charged photoreceptor, the light passes through the transparent charge transport layer and is absorbed by the charge generation material in the charge generation layer. The charge generation material which absorbed the light generates a charge carrier. The charge carrier is injected to the charge transport layer and transported in the charge transport layer along an electric field formed by the charge. The carrier neutralizes the charge on the surface of the photoreceptor, resulting in formation of an electrostatic latent image. In the functionally-separated photoreceptor, it is known that a combination of a charge transport material mainly absorbing ultraviolet light and a charge generation material mainly absorbing visible light is used, and such a functionally-separated photoreceptor that can satisfy the above basic properties is available.
Recently, the electrophotographic process is required to have a higher speed and a smaller size, and therefore the photoreceptor is required to have high reliability and durability to maintain high quality images even with long-term repeated use besides the above-mentioned properties.
Further, the photoreceptor receives various mechanical and chemical loads in the electrophotographic process. The chemical loads include ozone, nitrogen oxides and the like generated from a charger in the electrophotographic process. These ozone or nitrogen oxides adhere to a surface of the photoreceptor to cause a chemical reaction. The ozone oxidizes a binder resin or a charge transport material included in the photoreceptor. Therefore, a molecular chain of the binder resin is cut, and further an organic acid such as carboxylic acid is formed. In addition, the nitrogen oxides adhered to the surface of the photoreceptor are ionized by water on the surface thereof or water in the air to become a material having an electricity inductivity.
When the organic acids and the material adhere to the surface of the photoreceptor, the surface thereof has a lower resistance and an electrostatic latent image is broken. Consequently, when the latent image is developed, a flawed toner image is formed on the photoreceptor. In addition, the discharged products adhered on the photoreceptor typically increase the friction coefficient, resulting in an increase of a mechanical load by a cleaning blade onto a contact plate of the photoreceptor. Further, the above-mentioned cut of the molecular chain of the binder resin accelerates an abrasion of the photoreceptor. In addition, the ozone and nitrogen oxides are environmental problems as well.
Typically, corona chargers and contact chargers have been used as a charger in the electrophotography.
The corona chargers include a corotron method and a scorotron method including a grid, in which a corona discharge is made by applying a D.C. voltage or a D.C. voltage superimposed with an alternate current to a charge wire formed of tungsten or nickel suspended in the middle of a housing shielded with a metal plate to charge the photoreceptor. However, this method produces ozone or nitrogen oxides because a high voltage is applied to the charge wire. The products give an adverse effect not only on the environment but also the durability of the photoreceptor and the image quality produced thereby.
Recently, instead of this method, the contact charger has come into practical use for the purpose of low ozone and electric power. The contact charging method is a method in which a D.C. voltage or a D.C. voltage superimposed with an alternate current is applied to a charger having a resistance of from about 102 to 1010 Ω·cm and the charger is contacted to the photoreceptor upon application of pressure to charge the photoreceptor. Because this method is performed by discharging from a charger to a body to be charged according to the Paschen's law, charging starts by applying a voltage not less than a threshold voltage. Compared with the corona charging method, the contact charging method has a lower applied voltage. However, the discharge produces a small amount of ozone and nitrogen oxides.
As a new charging method, Japanese Laid-Open Patent Publication No. 06-003921 discloses a method of directly charging a photoreceptor. In this charging method, a photoreceptor has a low resistant charge-injection layer on its surface and a voltage is applied to a contact charger such as a charging roller, a charging brush and a charging magnetic brush to charge the photoreceptor. This charging method charges only a desired part of a surface of the photoreceptor to be charged because of not using a discharge. Therefore, in comparison with the conventional charging methods, the method has much less ozone and nitrogen oxides and uses less electric power. The charge-injection layer includes a metal oxide such as a tin oxide in the resin to decrease the surface resistance of the photoreceptor. However, the surface resistance of the photoreceptor largely changes due to an environment such as temperature and humidity, in which it is used, and the photoreceptor is not stably charged. Therefore, as a method of controlling the environment of an apparatus, installing a heater in the apparatus can be considered. However, when the heater is installed, the total electric power consumption of the apparatus increases.
Japanese Laid-Open Patent Publications Nos. 8-76559 and 9-26681 disclose a method of charging a photoreceptor, in which the photoreceptor is irradiated to generate a charge, which is transported to a surface of the photoreceptor by an outside electric field. Compared with the charge-injection method, this method has less ozone and nitrogen oxides, uses less electric power, and has less change of properties of the photoreceptor due to an environment such as temperature and humidity.
In the charging methods disclosed in Japanese Laid-Open Patent Publications Nos. 8-76559 and 9-26681, existing electrophotographic photoreceptors such as a multi-layered photoreceptor including a charge transport layer overlying a charge generation layer on an electroconductive substrate and a single-layered photoreceptor including a single photosensitive layer are used.
The multi-layered photoreceptor has to transport a positive (negative) charge to a surface thereof when charged and transport a negative (positive) charge thereto when writing a latent image. Thus, the charge transport layer has to have a capability of transporting both positive and negative charges. However, at present, a material that efficiently transports both of the positive and negative charges is not available. When the single-layered photoreceptor is used, both of the positive and negative charges can be transported. However, compared with the multi-layered photoreceptor, the single-layered photoreceptor has less optical sensitivity. In addition, because the same layer generates a charge when the photoreceptor is charged and when a latent image is formed in the single-layered photoreceptor, a time between the charge generation when the photoreceptor is charged and the charge generation when the latent image is formed is limited. Therefore, the single-layered photoreceptor can neither have a smaller diameter nor produce images at a higher speed.
Japanese Laid-Open Patent Publication No. 2001-183853 discloses an electrophotographic photoreceptor including a charge generation layer when a latent image is formed, a charge transport layer, a charge generation layer when charged and an electrode at a surface end thereof on an electroconductive substrate. This photoreceptor can solve the above-mentioned problems. When the photoreceptor is charged, an electric voltage is applied to the electrode at the surface end of the photoreceptor to form an electric field and transport the charge. In this method, to uniformly charge the photoreceptor, the most surface layer thereof has to have a low resistance. When such a layer is formed, a latent image formed thereon expands and a thin line and minute dot reproducibility deteriorate.
To charge the photoreceptor and form a latent image in a good condition, charge generation materials used in respective charge generation layers are preferably different and a combination of the respective charge generation materials is considerably essential.
Because of these reasons, a need exists for an electrophotographic photoreceptor producing high quality images without producing ozone and nitrogen oxides when charged.